1. Field of Invention
The present invention relates to a wind power generation technology. More particularly, the present invention relates to a method for low voltage ride-through (LVRT) in a DFIG (doubly fed induction generator) system.
2. Description of Related Art
With the increasingly acute energy crisis and environmental issues, countries all over the world are vigorously developing renewable energy businesses, such as wind power generation and solar energy generation. Taking the wind power generation as an example, the wind power installed capacity in China is developing rapidly, from a stall-regulated wind power system to a variable-speed and constant-frequency (VSCF) wind power system, and from a wind power system with a gear case to a direct-drive wind power system without the gear case.
However, with continuously increasing of the wind power installed capacity, after grid-connected power generation is implemented, the influence of the increased wind power installed capacity on the electric grid cannot be simply ignored any more. For example, in order to deal with the influence of the wind power generator set on the electric grid, many countries in Europe have established new rules regulating new requirements on the grid-connected wind power generation, such as controls of active power and reactive power, controls of voltage and frequency, controls of electric power quality and a function of low voltage ride-through. When the grid-connected wind power generator set meets these requirements, then even if the electric grid fails (such as, voltage drops), the connected grid still can be operated uninterruptedly, so as to provide active power and reactive power to the electric grid rapidly, and thus the voltage and frequency of the electric grid can be recovered and becomes stable timely.
Taking a DFIG as an example, when the low voltage ride-through is implemented, in general, an AC crowbar is coupled to the rotor side of the power generator in parallel, or alternatively, a DC chopper is coupled to a DC link in parallel, so as to respectively prevent the bus overvoltage and the rotor-side inverter overcurrent caused by the drop and recovery of the grid voltage. However, the AC crowbar or DC chopper described above mainly has the following disadvantages: (1) for a passive-mode AC crowbar, over-voltage protection and over-current protection can only be performed after the crowbar is switched in and drive signals from the rotor-side inverter are locked simultaneously, and the switching-in of the passive-mode AC crowbar allows a DFIG to absorb large amounts of reactive power from the electric grid for long term, which is harmful to the stability recovery of the electric power system; (2) during the period in which the grid voltage drops and recovers, an active AC crowbar can deliver reactive power or active power only after the rotor-side inverter is turned on, but the rotor-side inverter and the AC crowbar cannot work at the same, and thus the rotor inrush current and the bus overvoltage may occur again when a converter works again; (3) when being incorporated into the DFIG system, the existing DC chopper can only prevent the bus overvoltage of the converter but cannot prevent the over-current of the rotor-side inverter, thus resulting in an excessively large selection range of the rotor-side converter device; and (4) when the AC crowbar and the DC chopper are used concurrently, if the grid voltage drops, upon being switched to the AC crowbar in the DFIG system, the drive signals of the rotor-side inverter need to be locked, thus causing many inconveniences for flexibly controlling a transducer.
In view of this, it is an issue desired to be solved by those with relevant skills in this industry regarding how to design a novel protection circuit, in which the rotor-side inverter overcurrent of the converter can be prevented effectively while the bus overvoltage protection of the converter is implemented.